The dramatic increase in desktop computing power driven by intranet-based operations and the increased demand for time-sensitive delivery between users has spurred development of high speed Ethernet local area networks (LANs). 100BASE-TX Ethernet (see IEEE Std. 802.3u-1995 CSMA/CD Access Method, Type 100 Base-T) using existing category 5 (CAT-5) copper wire, and the newly developing 1000BASE-T Ethernet (see IEEE Draft P802.3ab/D4.0 Physical Layer Specification for 1000 Mb/s Operation on Four Pairs of Category 5 or Better Twisted Pair Cable (1000 Base-T)) for gigabit-per-second transfer of data over category 5 data grade copper wiring, require new techniques in high speed symbol processing. On category 5 cabling, gigabit-per-second transfer can be accomplished utilizing four twisted pairs and a 125 megasymbol-per-second transfer rate on each pair where each symbol represents two bits.
Physically, data is transferred using a set of voltage pulses where each voltage represents one or more bits of data. Each voltage in the set is referred to as a symbol and the whole set of voltages is referred to as a symbol alphabet.
One system of transferring data at high rates is Non-Return-to-Zero (NRZ) signaling. In NRZ, the symbol alphabet {A} is {−1, +1}. A logical “1” is transmitted as a positive voltage while a logical “0” is transmitted as a negative voltage. At 125 megasymbols per second, the pulse width of each symbol (the positive or negative voltage) is 8 ns.
An alternative modulation method for high speed symbol transfer is Multilevel Threshold-3 (MLT3) and involves a three-level system. (See American National Standard Information System, Fibre Distributed Data Interface (FDDI) Part: Token Ring Twisted Pair Physical Layer Medium Dependent (TP-PMD), ANSI X3.263:199 X.) The symbol alphabet {A} for MLT3 is {−1, 0, +1}. In MLT3 transmission, a logical “1” is transmitted by either a −1 or a +1 while a logical “0” is transmitted as a 0. A transmission of two consecutive logical “1”s does not require the system to pass through zero in the transition. A transmission of the logical sequence (“1”, “0”, “1”) results in transmission of the symbols (+1, 0, −1) or (−1, 0, +1), depending on the symbols transmitted prior to this sequence. If the symbol transmitted immediately prior to the sequence was a +1, the symbols (+1, 0, −1) are transmitted. If the symbol transmitted before this sequence was a −1, the symbols (−1, 0, +1) are transmitted. If the symbol transmitted immediately before this sequence was a 0, the first symbol of the sequence transmitted will be a +1 if the previous logical “1” was transmitted as a −1 and will be a −1 if the previous logical “1” was transmitted as a +1.
The detection system in the MLT3 standard, however, needs to distinguish between three levels, instead of two levels as in a more typical two-level system. The signal-to-noise signal ratio required to achieve a particular bit error rate is higher for MLT3 signaling than for two-level systems. The advantage of the MLT3 system, however, is that the energy spectrum of the emitted radiation from the MLT3 system is concentrated at lower frequencies and therefore more easily meets FCC radiation emission standards for transmission over twisted pair cables. Other communication systems may use a symbol alphabet having more than two voltage levels in the physical layer in order to transmit multiple bits of data using each individual symbol. In Gigabit Ethernet over twisted pair CAT-5 cabling, for example, five-level Pulse-Amplitude Modulation (PAM-5) data can be transmitted at a baud rate of 125 megabaud. (See IEEE Draft P802.3ab/D4.0 Physical Layer Specification for 1000 Mb/s Operation on Four Pairs of Category 5 or Better Twisted Pair Cable (1000 Base-T).)
Therefore, there is a necessity for a receiver capable of receiving signals having large intersymbol interference from long transmission cables. There is also a necessity for reducing the difficulties associated with digital equalization of signals with large intersymbol interference without losing the equalization versatility required to optimize the receiver.